Question

Which from following amino acids contains heterocyclic ring at a side chain (R) group?

• A. Threonine
• B. Histidine
• C. Cysteine
• D. Valine

Hint:

Histidine is the amino acid with an imidazole heterocyclic ring in its side chain.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Amino acids consist of a central alpha-carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain known as the R group.
A heterocyclic ring is a cyclic compound that has atoms of at least two different elements as members of its ring (e.g., carbon and nitrogen).
Step 2: Key Formula or Approach:
Evaluate the functional groups present in the side chain (R group) of each given amino acid to identify which one contains a heterocyclic structure.
Step 3: Detailed Explanation:
Let's analyze the R groups of the given amino acids:
(A) Threonine: Its R group is \(-\text{CH(OH)}\text{CH}_3\). This is an aliphatic, hydroxyl-containing side chain without any ring structure.
(C) Cysteine: Its R group is \(-\text{CH}_2\text{SH}\). This is a simple sulfur-containing aliphatic side chain with no ring.
(D) Valine: Its R group is an isopropyl group, \(-\text{CH}(\text{CH}_3)_2\). This is a branched aliphatic side chain containing no rings.
(B) Histidine: Its R group contains an imidazole ring. An imidazole ring is a five-membered planar ring containing three carbon atoms and two nitrogen atoms.
Since the ring contains both carbon and nitrogen atoms, it is classified as a heterocyclic ring.
Step 4: Final Answer:
Histidine is the correct answer as it is the only one containing a heterocyclic ring in its side chain.